Notes by dr. Claudio Italiano
Until the introduction of highly sensitive
methods for the TSH assay, the traditional method to confirm a clinical
diagnosis of hyper- or hypothyroidism was represented by the measurement of the
plasma concentration of T4 and / or T3, together with the determination of the
hormonal binding. To evaluate the serum concentrations of T4 and T3 and, when
indicated, the serum concentration of rT3, highly specific radioimmunoassay
methods are used. The normal values are: 60-150 mmol / I (5-12 mg / d1) for
T4, l-3 mmol / I (70-190 ng / dl) for T3 0.2-0.6 nmol / I (10-40 ng / dl) for
rT3. As previously mentioned, both the alterations of the hormone binding by the
plasma proteins and the changes in the rate of hormone secretion influence the
concentration of hormone in the blood.
However, only changes in hormone secretion cause alterations in the concentration of free hormone to equilibrium. 1 free hormone levels are better
correlated with the metabolic state than total hormone concentrations, more
realistically reflecting the rhythm of hormone production. The concentration of
free T4 (free T4, FT4) can be measured directly by dialysis at the equilibrium
of the serum enriched with tracer amounts of labeled T4. The percentage of
dialysable or free T4 is then determined and the product of this value for the
total T4 represents the FT4. However, because the dialysis technique is
impractical, it has been replaced by indirect tests. The expression "FT4
Estimated" (FT4E) may refer either to the free T4 fraction determined by
dialysis techniques or to the concentration of free T4 measured indirectly or
calculated. A traditional indirect method, the in vitro collection test, is a
method of assessing the binding of thyroid hormones; in this test the serum is
enriched with labeled T4 or T3 and is then incubated with insoluble particles,
such as resins or coal, which bind the free hormone (fixation of T3 on resin,
RT3U). The percentage of radioactive hormone captured by the particles varies
inversely proportional to the concentration of unoccupied sites and their
affinity for the hormone being used. RT3U can be expressed as a thyroid hormone
binding ratio (THBR) binding ratio, reducing the risk of confusing T3 fixation
with the T3 assay. In most cases the values of RT3U, THBR or FTE are
proportional to those of the percentage of FT4 and free T3 (FT3). This
proportionality reflects the fact that in normal serum T4 and T3 are mainly
attached to a common TBG binding site. Thus, the binding alterations generated
by an excess or by a TBG deficiency, or by an excess or by a deficiency of T4,
do not significantly influence the relationship between the intensity of binding
with T4 and with T3. Under these conditions, it is therefore possible to
calculate a free T4 index (FT4I) and a free T3 index (FT3I), respectively as a
product of RT3U and of total T4 and T3 concentrations. These indices are
proportional to the real concentrations of FT4 and FT3. As pointed out above,
there are situations marked by an increased plasma T4 binding in which the
binding entity of T4 to T3 is abnormal, as TBG is not the protein involved. In
these cases, more often the T4 binding is greatly increased, whereas the T3
binding is slightly increased or normal.
These alterations include familial disalbuminemic hypertiroxinemia, inherited as an autosomal dominant trait, in which the plasma concentration of a variant of albumin with a particularly high affinity for T4 has increased. As a consequence, serum T4 is considerably higher than normal, but according to the euthyroid state, FT4 is normal. Since RT3U does not reflect the increase in T4 binding intensity, the calculated values of FT41 have increased and often lead to a misdiagnosis of fireotoxicosis. These findings are observed when there is an increased binding of T4 by transthyretin or when circulating antibodies against T4 are present. In these cases, serum T4 is increased by an increase in its plasma binding and FT4 and the metabolic state are normal. Therefore, these situations are described as euthyroidism with hyperthyroxinemia, a term that implies the presence of hyperthyroidism not caused by an inherent thyroid disease. The mechanisms responsible for this clinical picture are variable and in some cases uncertain. The increase in total T4 does not appear to have any effect on the metabolic state, but hyperthyroidism may be misdiagnosed. Some conditions are associated with an increased thyroid secretion of T3, at least compared to that of T4. As a consequence, the serum T3 concentration is disproportionately high compared to that of T4. Apparently this is a consequence of the follicular cell hyperfunction, being found in all the varieties of hyperthyroidism and in the early stages of thyroid insufficiency, during which the gland is exposed to an increased stimulation by the TSH. Thus, the serum concentration of T3 and the resulting FT3I are generally higher than the corresponding T4 values for the diagnosis of hyperthyroidism. In initial hypothyroidism, on the other hand, the concentration of serum T3 and FT31 are often normal, despite serum concentrations of T4 and FT41 below the norm. As a consequence, the serum T3 concentration is not reliable for the diagnosis of hypothyroidism. The determination of rT3 in serum is useful for differentiating "low T3 syndrome" (see below) from true hypothyroidism; in the first case the concentration of rT3 is increased, while in the second it is generally lower than normal.
Although the assessments of the metabolic effects of thyroid hormones are of diagnostic importance, none of them is sufficiently sensitive, specific and easily carried out to allow routine use. In the past, the determination of oxygen consumption at baseline (basal metabolism) was the foundation for the diagnosis of thyroid diseases, but is currently only of historical interest. Several blood chemistry tests can be altered in patients with thyroid diseases. For example, serum concentrations of creatine phosphokinase and, more rarely, of lactic dehydrogenase and aspartate aminotransferase increased in hypothyroidism, while in hyperthyroidism they may be slightly decreased. These changes are not specific and it is important to be aware of them to avoid creating confusion with other diseases characterized by similar changes. Serum concentrations of testosterone binding globulin (testosterone-binding globulin, TeBG) of ferritin (iron binding protein) and angiotensin converting enzyme depend on thyroid hormones and are, therefore, increased in thyrotoxicosis; however, they have no use for the diagnosis of thyroid diseases. Increases in serum cholesterol concentration are common in primitive hypothyroidism, while low levels are common in thyrotoxicosis. The systolic time indexes, such as the pre-objective period and the pulse wave time, are prolonged in hypothyroidism and abbreviated to hyperthyroidism; they can be useful in monitoring replacement thyroid therapy in the elderly or in heart patients.
Baseline concentration of serum TSH is useful in the diagnosis of both subclinical and hypothyroidism. The subclinical picture represents a stage in the evolution of hypothyroidism, in which a structural or functional anomaly that alters hormone synthesis is compensated by the hypersecretion of TSH. In thyrotoxic states the serum TSH concentration is almost always low or indeterminable. While conventional radioimmunoassay methods were unable to distinguish between normal and subnormal values, immunoradiometric or chemiluminescent techniques using monoclonal antibodies are currently characterized by excellent sensitivity. Patients with thyrotoxicosis tend to have indostable levels "0.1 mU / 1), while in most normal subjects the range of values, using these dosages, is between 0.3 and 3 mU / l. These dosing methods therefore offer significant advantages compared to traditional radioimmunoassay methods and may be useful in confirming a diagnosis of both hypothyroidism and hyperthyroidism. In patients with TSH-induced hyperthyroidism, serum TSH concentrations are elevated absolutely or inappropriately compared to FT4 and FT3 values. This rare syndrome derives from both a secreting TSH adenoma and the resistance of the TSH secretion mechanism to negative feedback. exercised by T4 and T3. The determination of TSH in serum is the best means to distinguish untreated hypothyroidism of thyroid origin, in which the values are invariably high, from hypophysis or hypothalamic hypothyroidism, in which the levels of TSH are generally reduced or normal. Some patients with hypothyroidism of hypothalamic or pituitary origin are in a form of immunoreactive TSH, but lacking biological action. In these cases, serum TSH concentrations may be elevated rather than reduced. The thyrotropin-releasing hormone secretion stimulating hormone stimulation test (TRH) evaluates the functional status of the secretory mechanism of TSH and has diagnostic significance under different circumstances. Following injection of TRH into a normal subject, serum TSH begins to increase, reaches a peak between 20 and 45 minutes, then declines rapidly. The nature of the hypophyseal feedback mechanism is such that, with a normal hypothalamic-pituitary function, an increase in the response to TRH is expected when the thyrotropic cell detects a deficit of thyroid hormones (particularly of T3) and a reduced or absent response when the thyroid hormones are in excess. Thus, except for the rare cases of hypophyseal resistance to thyroid hormones, in which the responses are generally normal, thyrotoxicosis is invariably associated with a decreased or absent TSH response to TRH. Because of the extreme sensitivity of the TSH feedback inhibition mechanism, reduced responses to TRH are commonly observed in apparently euthyroid patients with serum T3 or T4 levels in the normal, but with an increased production of T3 or T4 by adenomas. self-acting toxicants or toxic multinodular goiters. A reduced response to TRH can also be observed in some euthyroid patients with Graves' disease.
Furthermore, responses to TRH are often reduced in older subjects, especially in males. Despite these exceptions, an absent or lower response to TRH is an excellent proof of the diagnosis of thyrotoxicosis. With the availability of ultra-sensitive TSH assay methods, the TRH test is now rarely used for this purpose. The TRH test is of little use even in the diagnosis of hypothyroidism. The response is increased in patients with primary hypothyroidism, but the magnitude of the increase is generally proportional to the increase in basal serum TSH. Some patients with hypophyseal hypothyroidism have lower-than-normal responses and some with TRH deficiency due to hypothalamic alteration have an almost normal response. Furthermore, in a quarter of patients with hypothyroid-hypophyseal hypothyroidism, basal TSH concentrations of serum TSH are normal or slightly elevated and the response to TRH is exaggerated, even if the measured serum TSH may not be bioactive. The thyroid suppression test is used to evaluate the normal homeostatic control of thyroid function. Under normal conditions, exogenous thyroid hormones suppress the hypophyseal secretion of TSH, resulting in a reduction of radioactive iodine uptake. In view of the current use of liotyrinine (100 mg / die for 7-10 days), the resulting decline in serum T4, such as radioactive iodine uptake, may be useful as a suppression index. A normal response consists in the reduction of radioactive iodine uptake to less than half the control value and a decrease in serum T4 at the lower or lower limits; a normal response to the suppression test is not compatible with a situation of hyperthyroidism and excludes the diagnosis because the liotironime suppresses the TSH (rather than the thyroid directly) and because the TSH is not measurable in most of the states of hyperthyroidism . In hyperthyroidism there is always an altered suppression test, regardless of the etiological cause; this may indicate a functional autonomy of the thyroid, the presence of an abnormal stimulus (not TSH) or an autonomous secretion of TSH. However, an altered suppression test is not patoc, nomonic of hyperthyroidism, being present in Graves 'disease after treatment and in about half of euthyroid patients with ophthalmopathy produced by Graves' disease. Because of the risk of undesirable effects of exogenous thyroid hormones, in particular of liotyrinine, in the elderly and in heart patients, the ultrasensitive TSH has almost completely supplanted the thyroid suppression test in the diagnosis of hyperthyroidism. Various tests that do not evaluate thyroid function are useful for defining the nature of thyroid disease or for planning therapy. For example, in the serum of most patients with Hashimoto disease and in many, with primitive thyroid hypothyroidism or with Graves' disease, elevated titers of thyroid peroxidase antibody (antiTPO), also known as antimicrosomal or antithyroglobulin antibodies, are insurmountable. In Graves' disease, the serum also contains antibodies to the TSH receptor. In general, these are able to inhibit the TSH receptor binding (TSH binding inhibitory immunoglobulins [TSH-binding inhibitory immunoglobulins, TBH]) and to stimulate the production of cAMP (immunoglobulins or thyroid stimulating antibodies [thyroidstimulating antibodies or immunoglobulins , TSAb or TSI]). The clinical utility of IST determination lies in the fact that the disappearance of these factors from serum, during a sky of antithyroid therapy, indicates the possibility of long-term remission of hyperthyroidism when the therapy is interrupted. In some patients, TBII-like antibodies do not have intrinsic stimulating effects, but block the response to endogenous TSH and deter hypothyroidism without goiter. Both the stimulating antibodies and those blocking the TSH receptor have the ability to cross the placenta and, consequently, to determine in the neonate, respectively, a hyperthyroidism (neonatal Graves disease) or a transitory hypothyroidism. Measurement of these antibodies during the last months of pregnancy allows to evaluate the probabilities of a possible development of the disease in the newborn. Some patients, more commonly those with autoimmune thyroid disease, develop antibodies circulating against T3, T4, or both. In these cases the radioimmunoassay of T3 and T4 provides falsely increased or reduced values in relation to the radioimmunoassay technique used, since the circulating antibody competes with the exogenous antibody for binding with the labeled ligand.
The actual concentration of the hormone, determined on serum samples, has increased due to the additional binding sites provided by the antibody; however, the hormone attached to it is not available to perform a metabolic action. In the case of the anti-T3 antibodies, which are the most frequent, the RT3U values are low because the endogenous antibody competes with the resin for binding to the labeled T3. These antibodies can be shown by adding to the serum the labeled hormone and demonstrating the binding of immunoglobulins to the labeled hormone. As in many other thyroid diseases, even in cases of differentiated thyroid carcinomas, release of the thyroglobulin is observed; consequently, the radioimmunoassays of the serum concentration of thyroglobulin, although not having any value in the diagnosis of thyroid carcinoma, are useful for assessing the adequacy of initial therapy and signaling the appearance of relapses or the dissemination of the disease; Endogenous antitireoglobulin antibodies can determine falsely reduced values. In patients with thyrotoxicosis, lower serum thyroglobulin concentrations, along with reduced values of radioactive iodine uptake, suggest the presence of a fictitious thyrotoxicosis. The scintigraphy allows the localization of the radioiodine or sodium pertechnetate accumulation sites [99'Tc]. This technique is useful to define areas of increased or reduced activity within the gland and to highlight a retrosternal goitre, the presence of ectopic thyroid tissue, the thyroid hemiagenesis and the functioning metastases of thyroid carcinoma. Thyroid ultrasound is also useful for differentiating between cystic and solid nodules. Because ultrasound provides accurate information on thyroid morphology without being invasive and apparently without damaging effects, it can be used to evaluate changes in thyroid volume and individual nodules over time and in response to treatment.